Data Sheet

LT1103/LT1105 Offline Switching Regulator

FEATURES
s s s s s s s s s s s

DESCRIPTION
The LT®1103 Offline Switching Regulator is designed for high input voltage applications using an external FET switch whose source is driven by the open collector output of the LT1103. The LT1103 is optimized for 15W to 100W applications. For higher power applications or additional switch currentflexibility, the LT1105 is available and its totem pole output drives the gate of an external FET. Unique design of the LT1103/LT1105 eliminates the need for an optocoupler while still providing ±1% load and line regulation in a magnetic flux-sensed converter. This significantly simplifies the design of offline power supplies and reduces the number of components which must cross the isolation barrier toone, the transformer. The LT1103/LT1105 current mode switching techniques are well suited to transformer isolated flyback and forward topologies while providing ease of frequency compensation with a minimum of external components. Low external part count for a typical application combines with a
, LTC and LT are registered trademarks of Linear Technology Corporation.

±1% Line and Load Regulationwith No Optocoupler Switch Frequency Up to 200kHz Internal 2A Switch and Current Sense (LT1103) Internal 1A Totem-Pole Driver (LT1105) Start-Up Mode Draws Only 200µA Fully Protected Against Overloads Overvoltage Lockout of Main Supply Protected Against Underdrive or Overdrive to FET Operates in Continuous or Discontinuous Mode Ideal for Flyback and Forward Topologies Isolated Flyback Mode HasFully Floating Outputs

APPLICATIONS
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Up to 250W Isolated Mains Converter Up to 50W Isolated Telecom Converter Fully Isolated Multiple Outputs Distributed Power Conversion Networks

TYPICAL APPLICATION
Fully Isolated Flyback 100kHz 50W Converter with Load Regulation Compensation
OPTIONAL OUTPUT FILTER 85VAC TO 270VAC MBR2045 10µH 5V 10A

+ + –
1000pF 100Ω BRIDGE RECTIFIER + LINEFILTER 220µF 385V 499Ω

220k 1W

!! GER ! DAN LTAGE! VO HIGH
1N4148

1.5KE300A 5W MUR150

+

*50V 3600µF

*OUTPUT CAPACITOR IS THREE 1200µF, 50V CAPACITORS IN PARALLEL TO ACHIEVE REQUIRED RIPPLE CURRENT RATING AND LOW ESR. WINDINGS FOR OPTIONAL ±12VDC OUTPUTS
5.25 5.20 5.15

BAV21 BAV21 1N4148 39µF 35V 18.7k FB GND 4.75k 1% 0.047µF 330Ω 0.047µF 0.1µF
LT1103 TA13

VIN

VSWLT1103

BUK426-800A

13k 1%

+

10Ω 15V VC OSC 390pF

+

1µF 25V

TRANSFORMER DATA: COILTRONICS CTX110228-3 L(PRI) = 1.6mH NPRI:NSEC = 1:0.05 NBIAS:NSEC = 1:0.27

VOUT (V)

Danger!! Lethal Voltages Present – See Text

U

U

U

+

50V 470µF

Load Regulation

5.10 5.05 5.00 4.95 4.90 4.85 4.80 4.75 0 1 2 3 4 5 IOUT (A)
LT1103 TA02

220VAC

270VAC

110VAC

85VAC6

7

8

9

10

1

LT1103/LT1105
DESCRIPTION
200kHz maximum switching frequency to achieve high power density. Performance at switching frequencies above 100kHz may be degraded due to internal timing constraints associated with fully isolated flyback mode. Included are the oscillator, control, and protection circuitry such as current limit and overvoltage lockout. Switch frequencyand maximum duty cycle are adjustable. Bootstrap circuitry draws 200µA for start-up of isolated topologies. A 5V reference as well as a 15V gate bias are available to power external primary-side circuitry. No external current sense resistor is necessary with LT1103 because it is integrated with the high current switch. The LT1105 brings out the input to the current limit amplifier and requires theuse of an external sense resistor. The LT1103/LT1105 have unique features not found on other offline switching regulators. Adaptive antisat switch drive allows wide ranging load currents while maintaining high efficiency. The external FET is protected from insufficient or excessive gate drive voltage with a drive detection circuit. An externally activated shutdown mode reduces total supply...